The term “inertial sensor” throughout the disclosure comprises an accelerometer, a gyroscope, and alternatively a geomagnetic sensor.
Personal navigation systems are being developed for use in various applications. For example, personal navigation systems may be useful for tracking and directing the movements of military personnel in military battlefield environments or military practice maneuvers, for tracking and directing the positions of SWAT personnel during anti-terror actions or hostage rescue actions, for tracking and directing the positions of firefighters at the scene of an accident or fire, and for indoor mapping through user's walking. Especially, with respect to the applications for firefighters, firefighters may get lost, missing or even lose their lives in the fire scene with thick smoke because of the lack of effective indoor navigation and tracking systems. Accordingly, there exists a strong demand for developing effective positioning navigation systems for indoor use, which can be applied in the fields of military battles, military practice maneuvers, anti-terror rescue actions, search and rescue in fire scenes, and indoor mapping, etc.
While the satellite-based navigation technology for outdoor use, such as Global Positioning System (GPS) technology, has been widely used, it is not effective for indoor navigation. Conventional navigation systems for indoor use are generally costly or inaccurate. For example, the installation and maintenance problems are substantial barriers to installation of the base of radio frequency markers within a building. In addition, poor reception of radio frequency navigation signals within a building due to signal block, reflection, attenuation or multi-path effect leads to low positioning accuracy and insufficient signal coverage. Furthermore, multiple story buildings generally require three-dimensional positioning, which pose additional obstacles for accurate positioning.
Another type of navigating system is an inertial navigation system (INS), which comprises a calculating unit and an inertial measurement unit (IMU) containing an accelerometer, a gyroscope and alternatively a geomagnetic sensor. The inertial navigation system continuously reckons the position, orientation, and velocity of a moving object from its initial position, orientation and velocity without external references. The advantage of such navigation method is that it requires no external signals like the GPS signals or other radio frequency signals. However, the biggest shortcoming is that it suffers from accumulative errors. The IMU measures the acceleration and angular velocity, then the acceleration is integrated into velocity and is again integrated into movement distance, the angular velocity is integrated into orientation variation. During the integral operation, small errors in the measurements of acceleration and angular velocity are accumulated, which is compounded into greater errors in position.
The above accumulative errors generated in the inertial navigation system can be greatly reduced by mounting the IMU to feet or shoe of a user to be periodically motionless relative to the ground during the user's walking and adapting Zero Velocity Update (ZUPT) algorithm. However, the accumulated heading errors cannot be eliminated. Small heading bias may be enlarged to a great error by a long distance. For example, a heading bias of 3° will be enlarged to an error of 5.2 meters by a distance of 100 meters, which equals to one or two rooms in the indoor environment.
The primary cause of the heading error is that the gyroscope of the inertial sensor has zero drift, that is, even when the input angular velocity of the gyroscope is zero, the output angular velocity is not zero. Zero drift is an important index to evaluate the performance of the gyroscope. For a MEMS gyroscope, its core sensitive components and processing circuits are prone to be affected by complicated ambient environments. For example, either of the environmental factors like temperature, electromagnetism, vibration, radiation, gravity anomaly, humidity and pressure may affect the performance of the core sensitive components and processing circuits of the gyroscope, and may reduce the output measurement accuracy. These environmental factors are not easy to be quantified or studied precisely. The coupling and accumulation of the errors caused by these factors may result in a total error with strong randomicity and lack of regularity. Therefore, even the strict use of standard gyroscope testing, calibrating, modeling and compensating method to process the errors caused by the above environmental factors will not improve the accuracy of the gyroscope effectively.
Nowadays, one major way to overcome the above zero drift problem is establishing a model for motion of the gyroscope and then compensating for bias of the gyroscope according to the model. However, the zero drift of the gyroscope is characterized by small nonlinearity, non-stationarity and slow time variability, which requires online real-time fitting modeling and parameter identification to achieve good compensation results, making the compensation more difficult in a real-time system.
Other sensors, such as GPS sensors or magnetic sensors are used to detect heading directions, so as to overcome the problem of accumulative heading errors caused by the zero drift of the gyroscope. Nevertheless, with respect to the indoor environment, the GPS signals may be blocked by buildings to be ineffective; the magnetic sensors may be unreliable due to magnetic field interferences caused by iron or other materials inside the buildings. Accordingly, IMU with high performance are required to solve the above problems. However, the size, cost and structural complexity of such IMU with high performance also set limits to the actual application environment, which results in raised system cost and increased size of the whole system even to an extent that dissatisfying the application requirements. For example, a MEMS gyroscope with zero drift of less than 1° per hour costs tens of thousands of RMB, while fiber optic gyroscope of higher performance is more costly and becomes too big in size to be mounted on feet or shoe.